Plastic tubs are typically a little flexible. In my DIY rack I had about a 1/4" gap (maybe a little smaller) and I had an escape. She was about 400-500g at the time and managed to squeeze out. It took me a week to find her, needless to say I put lids on to fill in the gap after I found her. They can squeeze through some small spaces and if the plastic tubs are even remotely flexible they can get through.